DIFFUSION ACTIVITY OF CARBON DURING POLYMORPHIC TRANSFORMATION OF STEEL AND TWO-STAGE LOW-TEMPERATURE CEMENTATION

Abstract

It was studied diffusive mobility of carbon’s atoms in the process of decarbonation steel У8 and cementation steel Ст3 in the temperature’s interval of polymorphic transformation. It was set that low temperature cementation (LTC) is possible only after a preliminary austenitization steel. In the state of polymorphic transformation due to the weakening of interatomic interaction, the formation of a two-phase structure of complexes, there is a decrease in the activation energy of carbon diffusion and a corresponding increase in the diffusion coefficient, which indicates the possibility of conducting LTC. The diffusion ability of carbon in LTC in a two-phase state was estimated using the value of the effective activation energy of diffusion taking into account the relative content of austenite and ferrite according to the «lever» rule. As a result of the studies performed, the following two-stage LTC method is proposed: the first stage – austenization (carburization) of the surface layer at a temperature of 920 °С for 20 minutes; the second stage – cementation in the temperature range 700…740 °С for 2 hours. Subsequent hardening and low-temperature tempering provide the required quality of the cemented layer by analogy with the traditional cementation process.